1. Field of the Invention
The present invention relates to an image forming apparatus to form an image on a photosensitive drum according to an electrostatic photographing process and, more particularly, to an optical scanner as an exposure unit to form an electrostatic latent image on a photosensitive drum.
2. Description of the Related Art
An image forming apparatus using an electrostatic copying process, for instance, a digital copying machine is provided with an image reading portion to read an image of an original document as a copying object and an image forming portion to form a copying image based on the image that is read via the image reading portion and output it on a paper as a recording medium.
The image reading portion has a first carriage including an exposure lamp for scanning in one direction while illuminating an original document and a second carriage to guide the image information of the document to a photosensitive drum of the image forming portion following the first carriage.
The image forming portion has a photosensitive drum to carry an electrostatic latent image corresponding to the image information of the original document read by the image reading portion, an optical scanner as an exposure device to form an electrostatic latent image by exposing the photosensitive drum charged to a specified potential by generating a light beam corresponding to image information, a transferring device to transfer a developer image formed on the photosensitive drum onto a paper, etc.
The optical scanner included in the image forming portion has such optical element groups as a light source to generate a laser beam that is modulated according to image information, a first optical system to convert this laser beam into a parallel beam or a focusing beam, a light deflecting device to scan the laser beam in the specified direction and a second optical system to form the deflected laser beam into an image on the photosensitive drum. These optical element groups are arranged at specified locations in the housing, adjusted and then, fixed to the bottom of the housing.
This housing has one reference surface that is defined by three surfaces. When adjusting an optical axis and a beam diameter so that the laser beam emitted from the light source forms an image at a specified location on the photosensitive drum, the housing of the optical scanner is set on an adjusting device using this reference surface as a reference. Also, when mounting an adjusted optical scanner to the main body of the image forming apparatus, the scanner is fixed to the housing of the image forming apparatus using the reference surface utilized in the adjustment as the reference.
Generally, an optical scanner has a light deflecting device fixed at the bottom of its housing and a laser beam is emitted in the direction of a photosensitive drum from the bottom side of the housing where the light deflecting device is fixed. In such a housing, one reference surface is provided on the outer surfaces at its bottom for use as a reference for setting an adjusting device and as a reference for mounting an optical scanner to the main body of an image forming apparatus. In this case, the surface of the housing that is set for an adjusting device is in accord with the housing surface which is set when mounted to the main body of the image forming apparatus.
However, in case of an optical scanner in such structure that the housing surface that is set to a device when adjusting is not in accord with the housing surface that is set when mounting to the main body of the image forming apparatus, there is such a problem as shown below. That is, in case of a housing constructed to emit a laser beam to a photosensitive drum from an opening at the side opposite to the bottom side where a light deflecting device is provided with the optical axis between them, if a reference surface is provided on the opening side only, an optical scanner is mounted to the main body of an image forming apparatus using this reference surface as a reference. However, to perform the optical adjustment by setting an optical scanner on an adjusting device using this reference surface as the reference, a very large scaled device will be needed.
As described above, one reference surface is sufficient enough for optical scanners in the former structure but it is difficult to use one reference surface for both adjusting and mounting on optical scanners in the latter structure. Accordingly, it is necessary to manufacture a housing of an optical scanner suited to each image forming apparatus, for instance, digital copying machine, laser printer, etc. and it is not possible to manufacture highly efficient general purpose housings. Because of this defect, cost of the main body of the housing and its manufacturing cost can be increased.